Chapter 1 Gate Appears on Terra
It is 905.M30, across the stars the Emperor and his sons campaign to bring the galaxy under humanity's rule. The Great Crusade is in its closing years as now there are few empires left which can challenge the Imperium. Mother Terra, the throne world, stands, untouched by the foul taint of the heretic, the mutant and the xenos since the end of the Unification Wars and the Chaos Solar Incursion.
Few thought it would ever be touched by malignant forces ever again or at least not when the power of the Imperium was rising so rapidly in the finishing battles of the greatest endeavor in mankind's history. None though, not even the Emperor or the false god things in the warp could have expected the appearance of...The Gate.
Terra here is a sprawling global hive city, but its skies are not choked with smog nor its streets caked in filth thanks to the reconstruction efforts that came after unification. Its people are not the Emperor fearing mob of the kind seen in another grim dark timeline. Suddenly, in the early morning of yet another day that seemed as if it would pass as any other, every single psyker of iota classification and above on the planet, from its deepest under hives to even the heavily warded City of Sight, felt a strange occurrence in the warp. Something they will later describe as a kind of "splash" in its immaterial depths. At that very moment, exactly one hundred kilometers from the Lion's Gate of the Imperial Palace, "it" appeared.
On one of Terra's widest avenues, a structure inexplicably materialized. Such an event should have been impossible on the most fortified planet in the known universe and yet, seemingly out of thin air, something or someone had brought what to all looked to be a temple made in the gothic style familiar to any Imperial citizen.
However, although Imperial forces had been caught by surprise, they had not been caught unprepared, for Terra would not be known as the most suicidal place to strike in the entire galaxy had its defenders been complacent in the intervening years between now and the beginning of the Great Crusade. Adeptus Arbites rapid response teams rushed to the site where the Gate had appeared, cordoning it off to all non-military personnel. Rhino and Repressor APCs with heavily armored Arbiters surrounded the white, marble and ornately decorated building while maintaining a constant watch over its entrance. Overhead, Arbites Valkyries circle, prepared to blast anything which violates the no-fly-zone declared above the cordon. The highest levels of the Imperium's government on Terra were made aware of the developing situation and all forces on Terra and in the Sol system, was placed on high alert.
Then, around one hour after the Gate had manifested upon the throne world, the Arbiters on watch duty at the entrance reported spotting a large mass of figures rapidly approaching. The Arbites Judge in command ordered all Arbites personnel around the Gate to prepare for imminent combat. Once the first being stepped out from the entrance, the first hostile army to step foot on Terra in centuries came into view.
Arbites cordon around the unknown construct
Arbites Judge Mathew Grigorovich had seen many things in his forty years of service enforcing the Lex Imperialis on humanities home world. From battling mutants in the few remaining under hives that had yet to undergo reconstruction with far too many eyes and limbs, to arresting simple smugglers trying to get illegal cargo on and off world. He was a true veteran member of the Imperium's primary law enforcement organ and now sixty years old, kept in the field by some light rejuvenate treatments.
Out of all the things he had seen on the throne world however, this was perhaps the strangest sight of them all. The first ranks of the mysterious army which had exited the just as, if not more mysterious Gate, were made up of strange ork like creatures which, according to what he knew and what his data slate was telling him, had not been encountered before by the Imperium. They were armed, even by the standards of that savage idiotic race except for perhaps the most feral of green skins, with crude weaponry. Most held wooden clubs that clearly hadn't even been sanded with others wielding iron daggers, inferior to even the most basic Imperial Army trench knife. Behind them stood what were clearly human infantry, organized into packed square formations with around a hundred men each. Their equipment was far better and standardized, but still left much to be desired even if you compared it to most feudal worlds which could be found in the Emperor's domain. They held wooden shields in one hand and a short sword in the other, their bodies protected by armor which covered their torsos and a helmet. Although he was certainly no tech-priest, he could tell the armor and blades these men wielded were made of a low carbon steel which even an auto-pistol used by an under-hive ganger could pierce. As far as he could tell none of the soldiers on the opposite side held a gun of any kind, not even a muzzle loading matchlock using black powder, though there were a few archers in their rear lines.
"What in the Emperor's name," he muttered before activating the vox amplifier in his Arbites issue carapace armor. "To the army which has exited the unknown construct, you are all intruding upon the throne world of his majesty the Emperor, Master of all Mankind. You have thirty seconds to identify yourselves or you will be fired upon," he declared to the opposing side. Upon hearing his last two words, all Arbiters in front of the Gate trained their weapons upon the strange xenos and human force while the turrets of the Repressor APCs turned to face the potential threats as pintle mounted heavy bolters on the Rhinos were primed. The Valkyries above continued to circle, vigilant for any other opposing forces that may emerge.
What followed Judge Grigorovich's declaration was a surprise, but also an outcome many in the Arbites contingent present had already expected and something much more predictable than anything else that happened so far on this Emperor damned day.
A horn was sounded on the other side and the ork like xenos at the very front charged.